1. Field of the Invention
The present invention relates to an apparatus and method for writing servo tracks to a disk drive to reduce the sensitivity of the disk drive to sources of position error signal (PES).
2. Description of the Related Art
Disk drives generally have one or more rigid disks on which information can be permanently stored in the form of magnetic transitions written onto and read from the disks. These transitions are written onto and read from a plurality of generally concentric data tracks that are located on the disk. The reading and writing is performed by a head that is positioned over the disk surface by a rotary actuator. A servo system is provided on the disks to enable the head to access, or to seek, a particular track. One such servo system is an embedded servo system, where the servo information is written into servo sectors, i.e., arcuate portions of the concentric tracks reserved for the servo information. The servo sectors are located between data zones, i.e., acurate portion of the concentric tracks reserved for user data. Servo information is written into servo sectors in a process called servowriting.
Servowriting is performed during manufacturing in a device called a servo track writer. The servo track writer is configured to receive a disk drive sub-assembly, called a head disk assembly (HDA), that comprises one or more disks rotatably mounted on a spindle motor, a dedicated head mounted on a head actuator for each disk, and electronics designed to control the spindle motor, the head actuator, and the head. Once positioned in the servo track writer, the servo information can be written to the disks. The servo writing process also establishes the radial density of the tracks, which is typically measured in tracks per inch (xe2x80x9cTPIxe2x80x9d). More specifically, the servo track writer employs a push pin which is inserted into the HDA to engage the actuator arm. A head positioner circuit actuates the push pin by constant increments to position the heads radially over the disk while writing the servo information into the servo sectors. The HDA electronics biases the actuator arm against the push pin to facilitate precise positioning of the heads.
As mentioned above, the head is positioned over the disk on a rotary actuator and moved by constant increments by the push pin during servowriting. One consequence of the rotary motion of the head over the disk surface is that the head approaches the data zones at varying angles from the inner portion of the disk to the outer portion. At some point in the middle of the disk, a longitudinal axis through the head and a tangent line to the track rotating beneath the head are parallel. This is called the zero skew angle position. By contrast, at points on either side of the zero skew position, there is a skew angle between the longitudinal axis of the head and the tangent to the track. The skew angle increases as the head moves closer to the outer circumference. As a result, the tracks written near the outer circumference of the disk are narrower and closer together (i.e., have a higher track density) than the tracks written near the zero skew position.
Another result of higher skew angle at the outer diameter and the constant incrementing of the actuator by the push pin is that the TPI of tracks near the outer diameter is higher than the TPI near the zero skew position. Increased track density at the outer diameter has been considered a benefit because outer diameter tracks contain a higher proportion of bits than other tracks on the disk. However, track following is much more difficult at the outer diameter tracks for several reasons. The heads are subjected to more turbulence than at other parts of the disk. Also, the disk itself is less stable because vibrations induced in the disk, known as flutter, are greatest at the outer diameter. These and other factors make track following more difficult at the outer diameter compared to other positions in the range of motion of the actuator. This phenomenon can be observed as increased position error signal at the outer diameter tracks compared to other tracks in the drive.
A need exists for an apparatus and method to reduce the sensitivity of the servo system, as measured by position error signal, to turbulence and other physical stresses at the outer diameter. Further, there is a need to provide more servo margin at the disk outer diameter while at the same time increasing overall track density.
One aspect of the present invention is a method for reducing position error signal in a disk drive. The disk drive comprises a recording head, a rotary actuator, a spindle motor, and a storage medium mounted on the spindle motor. The storage medium comprises an inner circumference, an outer circumference, and a magnetic-storage portion between the inner circumference and the outer circumference. The magnetic-storage portion has a radial width. The magnetic-storage portion comprises a first radial band proximate the outer circumference that includes about one-third of the radial width. The magnetic-storage portion comprises a second radial band proximate the inner circumference that includes about one-third the radial width. The magnetic-storage portion also comprises a third radial band between the first radial band and the second radial band that includes about one-third the radial width. The method comprises writing a first plurality of generally concentric servo tracks to the storage medium in the first radial band at a first average servo track density. The method further comprises writing a second plurality of generally concentric servo tracks to the storage medium in the second radial band at a second average servo track density. The method further comprises writing a third plurality of generally concentric servo tracks to the storage medium in the third radial band at a third average servo track density. The first average servo track density is no greater than the second average servo track density. The first average servo track density is no greater than the third average servo track density.
Another aspect of the present invention is a disk drive that comprises a rotary actuator, a recording head mounted on the rotary actuator, a spindle motor, and a storage medium mounted on the spindle motor. The storage medium comprises an inner circumference, an outer circumference, and a magnetic-storage portion between the inner circumference and the outer circumference. The magnetic-storage portion has a radial width. The magnetic-storage portion comprises a first radial band proximate the outer circumference that includes about one-third of the radial width. The first radial band includes a first plurality of generally concentric servo tracks. The first plurality of concentric servo tracks are written at a first average servo track density. The magnetic-storage portion comprises a second radial band proximate the inner circumference that includes about one-third the radial width. The second radial band includes a second plurality of generally concentric servo tracks. The second plurality of servo tracks are written at a second average servo track density. The magnetic-storage portion further comprises a third radial band between the first radial band and the second radial band. The third radial band includes about one-third the radial width. The third radial band also includes a third plurality of generally concentric servo tracks. The third plurality of servo tracks are written at a third average servo track density. The first average servo track density is no greater than the second average servo track density. The first average servo track density is no greater than the third average servo track density.
Another aspect of the present invention is a servo track writer that writes servo tracks on a storage medium of a head-disk assembly. The head-disk assembly includes at least one recording head mounted on an actuator. The storage medium comprises an inner circumference, an outer circumference, and a magnetic-storage portion between the inner circumference and the outer circumference. The magnetic-storage portion has a radial width. The magnetic-storage portion comprises a first radial band proximate the outer circumference that includes about one-third of the radial width. The magnetic-storage portion comprises a second radial band proximate the inner circumference that includes about one-third the radial width. The magnetic-storage portion also includes a third radial band between the first radial band and the second radial band that includes about one-third the radial width. The servo track writer comprises a clock head that writes a clock reference pattern to the storage medium, and that reads the clock reference pattern from the storage medium. The servo track writer also comprises a timing circuit that processes the clock reference pattern read from the storage medium and that generates a timing clock. The servo track writer also comprises a controller that processes the timing clock to determine the circumferential location of the recording head and that directs the recording head to write the servo tracks to the storage medium. The servo track writer also comprises a push pin, and a head positioner circuit. The head positioner circuit comprises a positioning mechanism that actuates the push pin in a step-wise fashion to impart a step-wise movement to the actuator. The step-wise motion is imparted by the push pin at a first average stepping increment when the recording head is over the first radial band. The step-wise motion is imparted by the push pin at a second average stepping increment when the recording head is over the second radial band. The step-wise motion is imparted by the push pin at a third average stepping increment when the recording head is over the third radial band. The first average stepping increment is also greater than the second stepping increment. The first average stepping increment is greater than the third stepping increment.
Another aspect of the present invention is a disk drive that comprises a rotary actuator, a recording head mounted on the rotary actuator, a spindle motor, and a storage medium mounted on the spindle motor. The storage medium comprises an inner circumference, an outer circumference, and a magnetic-storage portion between the inner circumference and the outer circumference. The magnetic-storage portion has a radial width. The magnetic-storage portion comprises a first radial band proximate the outer circumference that includes about one-third of the radial width. The magnetic-storage portion also comprises a second radial band proximate the inner circumference that includes about one-third the radial width. The magnetic-storage portion also comprises a third radial band between the first radial band and the second radial band that includes about one-third the radial width. A first plurality of servo tracks are written to the storage medium in the first radial band at a first average servo track density. A second plurality of servo tracks are written to the storage medium in the second radial band at a second average servo track density. A third plurality of servo tracks are written to the storage medium in the third radial band at a third average servo track density. The first average servo track density is no greater than the second average servo track density. The first average servo track density is no greater than the third average servo track density.
Another aspect of the present invention is a servo writer and disk drive combination. The disk drive comprises a recording head, a rotary actuator, a spindle motor, and a storage medium mounted on the spindle motor. The storage medium comprises an inner circumference, an outer circumference, and a magnetic-storage portion between the inner circumference and the outer circumference. The magnetic-storage portion has a radial width. The magnetic-storage portion comprises a first radial band proximate the outer circumference that includes about one-third of the radial width. The magnetic-storage portion also comprises a second radial band proximate the inner circumference that includes about one-third the radial width. The magnetic-storage portion also includes a third radial band between the first radial band and the second radial band that includes about one-third the radial width. The servo track writer comprises a clock head that reads a magnetic clock pattern in a clock track of the storage medium, and a timing circuit that processes the clock pattern and generates a timing clock. The servo track writer also comprises a controller that processes-the timing clock to determine the circumferential location of the head and that directs the head to write the servo tracks. The servo track writer also includes a push pin that engages the rotary actuator in the disk drive, and a head positioner circuit. The head positioner circuit comprises a positioning mechanism that actuates the push pin in a step-wise fashion to impart a step-wise movement to the actuator. The step-wise motion is imparted by the push pin at a first average stepping increment when the recording head is located over the first radial band. The step-wise motion is imparted by the push pin at a second average stepping increment when the recording head is located over the second radial band. The step-wise motion is imparted by the push pin at a third average stepping increment when the recording head is located over the third radial band. The first average stepping increment is greater than the second average stepping increment. The first average stepping increment is also greater than the third average stepping increment.